Miniature articles are difficult to handle, especially when precise registration of an article to a site is required. The problem is exemplified in the solid state electronics industry with respect to thin, rectangular or square, semiconductor chips having relatively flat, major surfaces. Such chips are handled during separation from a wafer, during testing and when they are assembled into a service package. Precise registration to a site is typically desired for testing and often demanded for bonding assemblies. A problem is that a top, active surface of most chips has delicate, electronic features constructed therein which are vulnerable to damage by mechanical contact of a pickup tool.
Early expedients for registering such chips to a site included vacuum powered probes which lightly contacted an active surface of a chip. Orientation of a chip was difficult to achieve and maintain and the contact sometimes damaged features in an active surface. Chips were typically bonded by applying a body of solder to a site, positioning a chip upon the solder and heating the site to reflow the solder. A problem was that the solder interfered with precise registration of a chip to a site. Another problem was that a resulting intermediate layer of solder sometimes slowed a desired drain of operating heat away from a chip.
A desirable expedient for handling a chip without contacting an active surface is to utilize a vacuum head having a cavity in a downwardly presented working face. Top edges of a chip become engaged by vacuum to inwardly and upwardly inclined surfaces on walls of the cavity to achieve and maintain a desired orientation. Such walls also provide sufficient lateral restraint that a pickup head may be utilized for improved bonding. For example, the head may be coupled to an ultrasonic vibrator to achieve bonding with or without externally applied heat and without solder. A problem is that registration is achieved by comparing lateral features of a chip to a site, and the cavity walls typically obscure at least one-half of a chip viewed along a workplane extending across the working face of a head.
Another problem is that solid state chips now range downwardly to sizes so small that major magnification and mechanical manipulation are often required for registration to a site. A problem with such magnification is that an operator's viewing field is restricted by a required fixing of a microscope (including at least one eyepiece) in space. The manipulation is performed while one is viewing through the eyepiece at a considerable height above the workplane of a head. One sees an oblique, foreshortened view of the head and the chip is typically, completely hidden from the viewing field. Consequently, a position of a chip in a cavity is estimated relative to viewable head features. Such estimated position is compared to a site by observing the viewable head features rather than by comparing the chip edges directly to a site. The registration achievable by such relative comparing is too imprecise for many of today's assembly operations.
Accordingly, it is desirable to develop new and improved pickup heads for registering articles, including solid state chips, to sites. Such heads should reveal at least some bottom edges of a chip, preferably extending to corners facing an operator for such registration. As applied to bonding apparatus, such heads should resist forces experienced in ultrasonically bonding a chip to a heat conductive site on a pedestal or other substrate.